1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, to a pneumatic tire in which not only heat build-up resistance and wear resistance are good but also controllability, traction and braking abilities on snowy, icy and wet road surfaces are excellent.
2. Description of the Related Art
Due to the regulation of a spiked tire, a studless tire, which satisfies the controllability, traction and braking abilities of tires on snowy or icy road surface (hereinafter, "snow/ice gripping ability"), and the controllability, traction and braking abilities of tires on the wet road surface (hereinafter, "wet skid resistance"), has been strongly desired. Various studies have been made to the tread patterns of the tire, rubber members of the tread or the like.
As an excellent example of improving the snow/ice gripping ability by the rubber members of the tire tread, a so-called foamed tire (U.S. Pat. No. 5,147,477) is well known. Here, a foamed rubber layer is provided at the rubber of the tread and a rubber composition, in which natural rubber/polybutadiene rubber and carbon black are mainly mixed, is used for the rubber layer. This foamed tire has excellent snow/ice gripping ability, and, in order to manufacture the tire, it has solved-a difficult technical drawback of controlling both vulcanization reaction and foaming reaction at the time of vulcanization.
In order to prevent hardening of the foamed tire in a low temperature area of snow or iced road surface, it is preferable that the amount of polybutadiene rubber is increased. However, wet skid resistance of the polybutadiene rubber is rather poor. Consequently, as is frequently seen in winter, in a case in which a snowy or icy portion and a wet portion exist on the same road surface, it is necessary to increase the wet skid resistance of the tire by effectively utilizing, for example, the way in which a suitable siping pattern is disposed on the tread of the tire. In view of these points, the inventors have done various studies to achieve higher levels of snow/ice gripping ability and wet skid resistance of the tire by improving the foamed rubber layer itself. Thus, the inventors have achieved the present invention which will be summarized hereinafter.